A remix is basically a song made by one artist that has been "reformatted" or "recomposed" to keep some parts of the original, but to add the personal flair and style of the remixer.
A list of dubstep remixes can be found here.
Remixes are typically credited as "ORIGINAL ARTIST - TRACK NAME (REMIXER, remix). So if someone were to remix a song it would end up looking like this format:
- Benny Benassi - Cinema (Skrillex remix)
- Black Eyed Peas - Rock That Body (Skrillex remix)
- La Roux - In For the Kill (Skream's "Let's Get Ravey" remix)
- Noisia - Machine Gun (16bit remix)